Conventionally, a transfer technique called a “multipath” has been known as a technique to improve use efficiency of a network. If this multipath is used, for example, a plurality of transfer paths (paths) can be used at the same time when transferring packets; and, therefore, loads in the network can be distributed. Furthermore, there is a known transfer technique using a plurality of shortest paths at the same time by combining the above-described multipath technique and a technique calculating and selecting the shortest paths from among a plurality of paths. This transfer technique using the plurality of shortest paths at the same time is particularly called “ECMP” (Equal Cost Multi-Path) among multipaths.
Meanwhile, since the plurality of paths are used at the same time as described above in the network environment which utilizes the multipaths or the ECMP, it is necessary to appropriately decide a path to be actually used from among a plurality of available paths. If the decided path is inappropriate, the transfer order of packets transferred from a source to a destination may be replaced with the order of arrival of packets which actually arrived at the destination, thereby causing a problem. This happens in a case, for example, where when it is intended to transfer packets A and B from the source to the destination in the order of packet A and then packet B and make them arrive at the destination in the order of packet A and then packet B and although packet A should be transferred through path A and packet B should be transferred through path B, packet A is transferred through path B and packet B is transferred through path A; and as a result, the order of arrival at the destination becomes packet B and then packet A.
Then, Patent Literature 1 discloses a technique that decides a path to be actually used from among a plurality of available paths based on a pair of a source address and a destination address (address pair). This address pair is, for example, a pair of a MAC address of a source (Source MAC address) and a MAC address of a destination (Destination MAC Address). In fact, when a network device such as a switch or a router receives a packet from the source, it calculates a hash value of the address pair included in a header of this packet and decides a path to be actually used based on the calculation result. Incidentally, there is a technique that decides the path to be actually used, based only on the address of the source.
Then, if this technique described in Patent Literature 1 is used, it is possible to solve the above-described problem of replacement of the order of packets transferred from the source with the order of packets which have actually arrived at the destination.